The present invention generally relates to optical recording and reading, and more particularly, to a near-field optical storage system and a multilayer optical medium.
Optical storage can be used to achieve high areal density data storage by using a tightly focused laser beam. For example, electro-optical data storage systems based on magneto-optical materials can be configured to produce an areal data density of up to or higher than about one gigabit per square inch. A monochromatic optical beam can be focused to a small spot by using an optical head with a large numerical aperture. This can produce a minimum spot size on the order of one wavelength due to the diffraction limit. The areal density of an optical storage device, in principle, is limited by this diffraction-limited spot size. The areal data density may be increased by reducing the spot size of a beam within the diffraction limit by using light sources of short wavelengths, such as lasers toward the blue end of the optical spectrum.
For a given wavelength, the area data density of an optical storage system can be increased by focusing an optical beam onto a flat surface of a solid transparent material with a high refractive index that is implemented in the optical head. The diffraction-limited focused spot size is hence reduced by a factor of the refractive index compared to the spot size in air.
In particular, a near-field configuration between the optical head and the optical medium may be formed by placing the optical head near the medium surface at a distance on the order of or less than one wavelength to effect evanescent optical coupling therebetween.
For example, the medium surface and the flat surface of the solid material may be typically spaced closer than one wavelength. U.S. Pat. No. 5,125,750 to Corle and Kino discloses a near-field optical recording system based on a solid immersion lens.
In a near-field configuration, the numerical aperture of the optical head can be greater than unity which is beyond the diffraction limit in air.
The present invention is embedded in an electro-optical data storage system in a near-field configuration. This system includes an optical train which has an optical head for coupling optical energy to and from a recording layer in an optical storage medium. In a preferred embodiment, the optical head is spaced from the surface of the medium by an air gap typically less than one wavelength in thickness. Hence, the optical coupling between the optical head and the optical medium is effected by both the optical propagation and evanescent coupling through the air gap.
An optical beam from the optical head is focused onto the medium and causes localized heating at and near the focused spot. This localized heating can modify certain properties of the interface of the optical head and the optical medium and thereby can cause distortion in the received signals. This may adversely affect the performance of the system. The recognition of such a problem is one aspect of the invention.
Another aspect of the invention is a multilayer structure of the optical storage medium that reduces the adverse effects of the localized heating. The multiple layers are preferably configured to substantially confine the heat within or near the recording layer to reduce the temperature change at the surface of the medium close to the optical head.
In one embodiment, a special capping layer may be formed on the medium surface to thermally insulate the optical head and the medium. The capping layer may be formed of a thick and optically transparent material with a low thermal conductivity. This reduces the thermal feedback from the medium to the optical head caused by the localized heating. A material comprising at least one of a diamond-like-carbon material, silicon nitride, silicon dioxide and others, for example, can be used to form the capping layer.
In another embodiment, one or more heat-dissipating layers having high thermal conductivities may be used to reduce the amount of thermal energy conducted to the medium surface near the optical head. Such a heat-conducting layer may be formed in the medium either between the optical head and the recording layer or on the other side of the recording layer. When such a thermally conductive layer is formed between the interface and the recording layer, an optically transparent material, such as a layer of aluminum nitride, a thin transparent film of gold or silver, can be used.
In yet another embodiment, one or more capping layers may be combined with one or more heat-dissipating layers to further reduce the surface heating near the optical head.
These and other aspects and advantages of the present invention will become more apparent in light of the following detailed description, the accompanying drawings, and the appended claims.